Most likely, what they mean is.
They have relationships with the sites, where the sites will let them pay money for a backlink, and the SEO agency just put money on top. A lot of people who sell guest posts here do the same thing.
I.E I can contact a website asking for a backlink, this website will say "Sure $40" and then I come onto BHW and say "$120 for this backlink"
That's how simple some guest posting services work on here.
If you are looking for paid backlinks, my best advice to you, and I seriously mean this, is contact the owners of the site yourself. They will reply, and they will most likely charge you for content, but it's going to be a lot cheaper than anyone you buy off.
But my other advice to you is, you seem to think DA is worth a price. Do not buy backlinks with your current knowledge, you will throw your money away buying it based on DA, that's not how you look for quality in backlinks by any means. It's literally throwing money into a fire.